In a gas turbine engine, a compressor produces compressed air for a combustor that produces a hot gas flow that is passed through a gas turbine, where the gas turbine drives the compressor and a fan such as in an aero gas turbine engine. The gas turbine must withstand the highest temperatures within the engine. An efficiency of the engine can be increased using a higher gas flow temperature into the gas turbine.
A typical gas turbine includes a rotor with a number of turbine rotor blades attached. A rotor blade would have an attachment end such as a fir tree attachment or a dove tail attachment that slides within a slot formed on an outer surface of the rotor disk. The blade attachment and the disk slot must be machined to high precision in order to minimize a gap formed between the blades and the slots. For a larger engine, these gaps are relatively small and thus any leakage across the disk has a minimal effect. However, for a smaller engine such as one used for an unmanned aero vehicle or UAV, the relative size of a gap to the disk would be large and thus the leakage flow would be critical. For this reason, a gas turbine is typically formed as an integrally bladed rotor (IBR) in which the rotor blades and the rotor disk are all formed as a single piece to eliminate any gaps. A typical IBR for a small gas turbine engine is cast using the investment casting process in which a ceramic core is used in a mold in which liquid metal is poured. Some machining of the IBR can even be used. One disadvantage of the single piece IBR is that the rotor blades and the rotor disk are made from the same material.
Single crystal rotor blades provide superior high temperature creep strength in a temperature range of 1,400 F. and are considered preferred for turbine airfoil applications. However, attempts to use single crystal airfoils in integral turbine blades disks and nozzles have been hampered by inadequate grain boundary strength between adjacent single crystal airfoils, which necessitated unacceptably high manufacturing cost and high costs for casting and machining individual airfoils and bonding the individual airfoils into a bladed ring or nozzle.
A nickel based superalloy, or high-performance alloy, is an alloy that exhibits several key characteristics: excellent mechanical strength, resistance to thermal creep deformation, good surface stability, and resistance to corrosion or oxidation. The crystal structure is typically face-centered cubic austenitic. Examples of such alloys are Hastelloy, Inconel, Waspaloy, Rene alloys, Incoloy, MP98T, TMS alloys, and CMSX single crystal alloys.
Dual alloy blisks composed of a cast equiaxed superalloy blade ring bonded to a high-strength disk alloy are also prior art. However, the cast equiaxed airfoils limit the temperature capability and performance of the turbine. Single crystal superalloy can maintain metal temperature capabilities at temperatures of up to 100 degrees F. or more above the maximum temperature ranges of equiaxed materials.
U.S. Pat. No. 6,969,240 issued to Strangman on Nov. 29, 2005 discloses a lightweight high temperature bladed turbine disk intended for use in gas turbine engines. The bladed disk comprises a cast integral ring of single crystal airfoils with the primary and secondary crystallographic orientation being the same for each airfoil. Low-angle mismatch boundaries are present in the endwalls that couple adjacent airfoils. The cast ring of single crystal turbine blades is diffusion bonded to a high strength equiaxed disk. The resulting single crystal bladed disk is endowed superior performance, temperature capability, and lower weight and cost, relative to conventional turbines composed of individually cast single crystal blades, which are mechanically inserted into machined slots in the disk, or lower strength cast equiaxed blade rings that are diffusion bonded to a high strength turbine disk.